The invention relates to a passive infrared detector having a first sensor for generating an infrared signal representative of the difference in temperature between a heat source and its environment, a second sensor influenced by the ambient temperature of the detector, and an evaluation circuit for processing the infrared signal. The evaluation circuit has a temperature compensation for influencing the sensitivity of the detector as a function of said ambient temperature. The amplitude of the infrared signal is approximately proportional to the difference in temperature between the intruder and objects present in the background of the monitoring area which is hereinafter referred to as the background temperature. In actual fact the infrared signal corresponds to the Stefan Boltzmann Law, according to which the total radiation of the black body over all wavelengths per cm2 is proportional to the 4th power of the absolute temperature of the body. The sensitivity or detection range of passive infrared detectors is thus largely dependent on the background temperature, which means that the sensitivity decreases as the difference in temperature decreases, which is the case when the background temperature approximates the body temperature of the intruder, for example, in hot or tropical regions.
If one assumes that a space normally has a homogeneous temperature distribution, so that the background temperature is approximately identical to the ambient temperature of the detector and changes synchronously with it, the second sensor then delivers not only information on the ambient temperature but also on the background temperature. The second sensor thus opens up the possibility of recognizing an increase in background temperature to body temperature, and therewith associated reduction in the contrast in temperature between an intruder and the background, and amplifying the infrared signal as a function of the ambient temperature. Alternatively, the amplification of the infrared signal can remain unchanged and the alarm threshold of the detector can be changed appropriately.
Such a detector is described in U.S. Pat. No. 4,195,234 and has a constant detection sensitivity. However, when the ambient temperature exceeds the body temperature of the intruder, the amplification of the infrared signal is increased or the alarm threshold reduced. Also, when the body temperature drops below the ambient temperature, the detection sensitivity does not remain constant. These circumstances constitute undesireable drawbacks of the aforesaid detector.
In U.S. Pat. No. 5,629,676 a passive infrared detector is described, the sensitivity of which is designed to remain substantially constant even when the ambient temperature exceeds human body temperature. This aim is achieved in that after the minimum contrast in temperature has been exceeded, when intruder and background have approximately the same temperature, the sensitivity of the detector is reduced. The second sensor is usually arranged on the detector absorber plate provided inside the detector, and does not measure the background temperature or strictly speaking, even the temperature in the environment of the detector, but the temperature inside the detector. This can cause a mismatch of the sensitivity to occur, owing to a warm or cold draught at the site of the detector, because the detector heats up or cools down too much or too quickly compared with the background. This mismatch can lead to a reduction in the robustness of the detector rendering it susceptible to parasitic inductions, such as, for example, white light or EMC interferers and such.
The object of the present invention is to provide a passive infrared detector of the kind described above, but in which a temperature compensation mean has the effect of minimizing detector false alarms. This object is achieved according to the present invention by designing the temperature compensation in such a way that changes in the ambient temperature do not directly influence the sensitivity of the detector. The second sensor is preferably formed by a temperature sensor arranged inside the detector.
In a preferred embodiment of the detector according to the invention the influencing of the sensitivity of the detector takes place only after a delay. The delay is preferably effected when an increase in the ambient temperature would cause an increase in the sensitivity of the detector. The delay is different for an increase or decrease in the ambient temperature and/or above and below a minimum value of the difference in temperature between the heat source and the environment. The delay is preferably of a duration dependent on parameters, such as the speed of the change in ambient temperature, and/or by the absolute temperature. The delay may take place by electronic means or by heat insulation of the second sensor or of the component influenced by the ambient temperature. By delaying the influencing of the sensitivity of the detector, short local temperature variations of the detector (or in its direct environment) will not influence the sensitivity of the detector, and the temperature compensation will depend substantially on the course of the background temperature.
In another preferred embodiment of the detector according to the present invention, influencing of the sensitivity of the detector takes place as a function of the speed of the change in the ambient temperature. Preferably, when a presettable first value of the speed of the change in temperature is exceeded, the temperature compensation is switched over from a first to a second mode, and back to the first mode only after a drop below a second value of the speed. For example, the temperature compensation is activated in the first mode and deactivated in the second. Taking into account the speed of the change in ambient temperature has the advantage that abnormally fast temperature changes are suppressed and cannot lead to false alarms owing to unnecessarily increased sensitivity of the detector.